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[ATM] PVC fitting mirror-cell.
> PVC and silicone RTV do not mix. The bond if you are extremely lucky and do
> not stress of move the items may last a week or so.
Four years ago, our group built five "Bathroom Telescopes" using 6" f8 mirrors. The main
tube was a four foot long section of 8" foam cored (insulated) PVC Schedule 40 sewer pipe.
The trunions were made up from 4" PVC pipe caps cut and sanded to fit the OD of an 8"
PVC pipe coupling. The coupling had a flange inside, which we removed with grinding tools
so we could slide it up and down the OTA to balance the assembly. The coupling was slit
length wise with a hack saw and a pair of large hose clamps are used to tighten the coupling
against the OTA, after the eye piece is rotated into positon.
The trunions fit inside a pair of "V" notched wood rails like most Dobsons.
The mirror was glued to a single conventional toilet "riser" using white GE bathtub silicone
based caulking. We attached three right-angle brackets and rubber bushings to the riser
and used those to fix the cell to the OTA. Springs and wing nuts between the riser and the
right-angle brackets, provide the needed collimation.
These were the first scratch built telescopes any of us had ever completed. We didn't know
we weren't supposed to attach mirrors to PVC with silicone. Four years later, all five scopes
are still in regular service and the mirrors still cling for dear life to the risers without loss of
collimation.That they are terrible as telescopes has to be acknowlged simply because the
bearings lack Star ebony and the base rotates on 12 inch phono records. Movement in any
axis is judder of small "leaps".
Five telescopes is certainly not a "quarum" but the fact that they are all working should tell
you something. Perhaps it's all 'beginners luck'. Perhaps one day the mirror will be rattling
around inside but I rather doubt it. It's not that hot or humid inside the tube to affect the
silicone (heat and moisture are what cure silicone)
Nice job you did with common materials. Dobson would be proud!
Art Bianconi
>What are the experiences of the list with silicone and PVC?
>
>Kurt
>Nixa, MO
>At 09:19 AM 4/17/04 -0500, Danny F. Barnes wrote:
>>Hi there,
>>
>>New to list and ATMing, starting with a 6" reflector. When it came to the
>>mirror cell, I made something out of PVC fittings, toilet flange
>>floor-plate things. Seems like it will work, has this been done before,
>>and if so, has it worked out? Is the PVC stable enough in varying temps
>>to keep the mirror in collimation?
>>
>>Planning on attaching the mirror to the cell with silicon.
>>http://www.geocities.com/dannybarnes@sbcglobal.net/Desktop.html
>>>>______________________________________________
>>ATM mailing list http://www.atmlist.net/
>>
>>
>>
>>
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Message: 5
Date: Sun, 18 Apr 2004 03:20:21 -0400
From: "Lawrence Sayre" <lsayre@neo.rr.com>
Subject: Re: [ATM] Secondary Cage Sizing?
To: "MATTHEW SIMMONS" <msimmonsmcse@msn.com>, "The Amateur Telescope
Makers List" <atm@atmlist.net>
Message-ID: <opr6m5b7fzs5tvhu@smtp-server>
Content-Type: text/plain; format=flowed; delsp=yes;
charset=iso-8859-15
On Sat, 17 Apr 2004 20:49:58 -0700, MATTHEW SIMMONS
<msimmonsmcse@msn.com>
wrote:
> So I have a design conundrum here....My specs are as follows:
>
> 13.1" f4.5
> 2.6" (?)
> .25" focuser board
> 1.55" Moonlite Crayford +.25" racked out
>
> So, according to K&B, my secondary cage should be ~13.75" and my
> illuminated FOV for a 2" ep sizes the secondary at just over 2.6" So far
> so good. However, NEWT says that I will vignette the 75% ray at front
> aperture. If I fool with the numbers in NEWT to prevent any vignetting,
> it will take a 14.75" secondary cage. However, when I run those numbers
> through the K&B equations, my fully illuminated FOV requires a 2.73"
> secondary.
>
> So, should I care more about not undersizing the secondary or vignetting
> the 75% ray at the front aperture? Which is worse for the overall
> performance of the scope?
>
>
>
> Matt Simmons
>
To avoid all possible vignetting at the front cage ring, just make sure
that your cage ID is at least the dia. of your mirror plus the field stop
diameter of the widest field stop eyepiece you ever plan to use in the
scope. For a totally maximized example:
13.1" = 332.74 mm aperture
46 mm = widest possible field stop for any 2" eyepiece
332.74 + 46 = 378.74 mm
378.74/25.4 = 14.91" ID rings for the secondary mirror cage
Using 9.505" as diagonal to the focal plain distance (based on a cage ID
of 14.91"), the excellent Mel Bartels Diagonal Calculator
( http://members.efn.org/~mbartels/tm/diagonal.htm ) suggests that a 2.6"
diameter secondary mirror will just ever so slightly vignette the
outermost 10% FOV visually for a 46mm field stop eyepiece. A drop-off of
up to 70% illumination at the extreme field edge is considered "visually"
imperceptible, and the "Diagonal Calculator" gives 68% illumination at the
outermost edge here. The 31mm Nagler has only a 42mm field stop, and the
Wide Scan III maxes out at 44mm. The only known eyepieces with 46mm field
stops are 40mm or more in focal length, so in the real world you (assuming
you are an average person) will not visually be able to perceive any drop
off at all (of more than 70%) in edge of field illumination with any
eyepiece available for actual use with your F/4.5 scope. You can go to
14.75" for the upper cage ID if the 31mm Nagler is to be considered, so
NEWT is right on the money here.
The only thing that happens if the cage rings are designed at 13.75" as
K&B suggest is that effectively you will loose about 1.1 inches of
aperture with your absolutely widest TFOV (largest field stop) eyepiece.
13.75" minus 1.73" (44mm field stop dia.) equals an aperture of 12.02
inches. The effect here will not be visible as edge of field vignetting
(as it would be if you went much smaller than 2.6" for your secondary
mirror), but only as a waste of (bought and paid for) aperture. However,
any other of your likely eyepieces in your collection will be 100% fine
with a 13.75" cage ID, as they will not loose aperture due to their far
smaller field stops.
It's always a matter of compromise in the end. Since most viewing will be
done with your smaller FL eyepieces, going with K&B for cage ID isn't a
bad choice overall when weight and size savings (and balance factors) are
considered. However, K&B uses the same old simple and incorrect formula
for deriving the 2.73" secondary mirror here that most books use. The
simple book formula always gives you a slightly larger secondary mirror
than is truly necessary. Mel Bartels Diagonal Calculator uses the far
more complex true formula (that being the same formula that the program
SEC.EXE uses, as well documented in an old Astronomy Magazine article), so
it is to be trusted over K&B, etc...
Lawrence Sayre
--
-----------------------------------------------------------------------
My philosophy, in essence, is the concept of man as
a moral being, with his own happiness as the moral
purpose of his life, with productive achievement as
his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
----------------------------------------------------------------------
------------------------------
Message: 6
Date: Sun, 18 Apr 2004 09:56:39 +0100
From: "Chris Rowland" <chris_group_mail@dsl.pipex.com>
Subject: Re: [ATM] PVC fitting mirror-cell.
To: <atm@atmlist.net>
Message-ID: <00ec01c42523$10ad6a60$97725651@bella>
Content-Type: text/plain; charset="iso-8859-1"
I have the secondary of my 8.75" Dob held to a PVC pipe with Silicone. It
has been there for over 3 years now. It is stored pointing up so the glue is
in tension all the time.
The secondary holder is made from a piece of 40mm PVC waste pipe with one
end cut at 45 derees and the other end square. The square end is glued to an
aluminium disk that holds the collimation screws, the 45 degree end has the
secondary glued on. All this is held together with RTV silicone, so I have
Al to PVC and PVC to glass glued joints.
The uPVC is sanded and I was very careful to degrease it - with surgical
spirit.
The primary mirror cell is made of plywood, varnished, and the mirror
attached with RTV.
If I were you I would try it. Sand a couple of spare pieces of PVC, clean
them and glue them together with a gap, as you will for the mirror. Wait a
few days then try to get them apart.
Chris
----- Original Message -----
From: "Charles Mitchard" <charlesmitchard@iinet.net.au>
To: <atm@atmlist.net>
Sent: Sunday, April 18, 2004 7:52 AM
Subject: Re: [ATM] PVC fitting mirror-cell.
> PVC and silicone RTV do not mix. The bond if you are extremely lucky and
do
> not stress of move the items may last a week or so.
> Definitely do not use as the only fixing for the mirror or secondary if
> using PVC or in general most other plastics.
> As Kurt states, it peels off at the slightest excuse with no adhesive
> properties.
> Charles
>
>
> >What are the experiences of the list with silicone and PVC?
> >
> >Kurt
> >Nixa, MO
> >
> >
> >
> >
> >At 09:19 AM 4/17/04 -0500, Danny F. Barnes wrote:
> >
> >>Hi there,
> >>
> >>New to list and ATMing, starting with a 6" reflector. When it came to
the
> >>mirror cell, I made something out of PVC fittings, toilet flange
> >>floor-plate things. Seems like it will work, has this been done before,
> >>and if so, has it worked out? Is the PVC stable enough in varying temps
> >>to keep the mirror in collimation?
> >>
> >>Planning on attaching the mirror to the cell with silicon.
> >>
> >>Images are here:
> >>http://www.geocities.com/dannybarnes@sbcglobal.net/Desktop.html
> >>
------------------------------
Message: 7
Date: Sun, 18 Apr 2004 09:15:40 -0400
From: "Lawrence Sayre" <lsayre@neo.rr.com>
Subject: Re: [ATM] Secondary Cage Sizing?
To: "MATTHEW SIMMONS" <msimmonsmcse@msn.com>, "The Amateur Telescope
Makers List" <atm@atmlist.net>
Message-ID: <opr6nlsevzs5tvhu@smtp-server>
Content-Type: text/plain; format=flowed; delsp=yes;
charset=iso-8859-15
On Sun, 18 Apr 2004 03:20:21 -0400, Lawrence Sayre <lsayre@neo.rr.com>
wrote:
Correcting my earlier post somewhat:
If you restrict your cage ID to 13.75" with a 13.1" aperture, you will
suffer some degree of aperture loss with every Nagler eyepiece until they
get to 11mm FL or less. I had earlier stated that likely only a 31mm
Nagler or a 30mm Wide Scan III would loose aperture due to this narrow
upper cage ID.
I believe that what K&B does actually say is to add 1" to your aperture
for your secondary cage ID. As I stated earlier (and NEWT confirms),
14.75" is actually required, but 14.1" (aperture + 1") would be an
acceptable weight and balance compromise ID. 13.75" might be a bit too
restricting here.
BTW, here is the proof for needing a 14.75" minimum cage ID with the 31mm
Nagler:
TFOV max = True (maximum) field of view
TFOV max = FS_eyepiece/FL_scope * 180/Pi
TFOV max = 42mm/(58.95" * 25.4mm/inch) * 57.29578
TFOV max = 1.607 degrees
1.607/2 = 0.8035 deg. (FOV for each extreme side of the mirror to contend
with while not vignetting)
Scope overall length to top of cage assumption: Mirror FL = top of cage
height = 58.95"
(for greater precision, substitute the actual distance from top/center of
the mirror to top of cage here)
(TAN 0.8035 deg) * 58.95" = 0.82675"
0.82675" * 2 = 1.6535" (bringing the two extreme sides of the mirror back
together at this juncture)
13.1" aperture + 1.6535" = 14.7535" cage ID for no vignetting (or just
plain tube ID for a conventional scope)
*** Please notice that 1.6535" is exactly the field stop diameter of the
31mm Nagler eyepiece in inches instead of mm. Therefore, in my first post
I simply went straight with "aperture plus eyepiece field stop = tube
ID". This simple truncation of the above equations holds true for any
eyepiece one might start with.***
Lawrence Sayre
--
-----------------------------------------------------------------------
My philosophy, in essence, is the concept of man as
a moral being, with his own happiness as the moral
purpose of his life, with productive achievement as
his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
----------------------------------------------------------------------
------------------------------
Message: 8
Date: Sun, 18 Apr 2004 09:12:47 -0500
From: "Jerry B. Hillman" <JBHillman@ev1.net>
Subject: Re: [ATM] PVC fitting mirror-cell.
To: <atm@atmlist.net>, "Danny F. Barnes" <dannybarnes@sbcglobal.net>
Message-ID: <001e01c4254f$3b05daa0$ff67fea9@ev1.net>
Content-Type: text/plain; charset="iso-8859-1"
Hi Danny,
Your bearing size should be okay. I use 4" bearings on my 10" and really
like how well it works. I also like your PVC flange mirror cell. Very
inventive and very useable. PVC is more stable than wood or metal in my
experience, and will outlast wood or metal by about 1000 years. If your
mirror is thin (ie 5/8" or less) you can attach plop type flotation bars to
the flange and still have a workable and good looking mirror cell.
Congrats on pushing mirror cell design to new levels. I love it when people
dare to think outside of the box.
Clear skies, Jerry
------------------------------
Message: 9
Date: Sun, 18 Apr 2004 09:22:58 -0500
From: "Jerry B. Hillman" <JBHillman@ev1.net>
Subject: Re: [ATM] PVC fitting mirror-cell.
To: <atm@atmlist.net>, "Danny F. Barnes" <dannybarnes@sbcglobal.net>
Message-ID: <004b01c42550$a68baec0$ff67fea9@ev1.net>
Content-Type: text/plain; charset="iso-8859-1"
Hi Danny,
Just a note about PVC and silicone adhesives. The posts to the list are
correct. PVC and silicone do not bond easily. However, attaching something
to the flange, (perhaps a washer about 5/8" in diameter with a small wood
screw set as flush as possible) at each point you want to put the RTV would
solve that problem neatly, and permanently.
Just a thought. I am sure you can solve this problem without difficulty.
Clear skies, Jerry
------------------------------
Message: 10
Date: Sun, 18 Apr 2004 13:07:17 -0400
From: "aneves" <aneves@snowsoft.biz>
Subject: [ATM] Machine Polishing
To: <atm@atmlist.net>
Message-ID: <200404181307.AA281477254@snowsoft.biz>
Content-Type: text/plain; charset=us-ascii
Hi,
What is the correct way to spherical polish a mirror on a machine with a sub diameter tool.
Every method I try I end up with a hole in the middle! Could some of your more experienced
on machines give me some hints! How do the pros do it? I am sure they must be able to pull
a perfect sphere on a machine.
I am doing an 8" with a 6" tool, 2" stroke offset by 2" to the right.
Regards,
Alfredo
------------------------------
Message: 11
Date: Sun, 18 Apr 2004 10:30:08 -0700
From: "Bob May" <bobmay@nethere.com>
Subject: Re: [ATM] plaster tools or something like it
To: "atml" <atm@atmlist.net>
Message-ID: <005501c4256a$cc042000$6d823f42@p120>
If you already have a plentiful supply of it, go and use it and see what
happens. I suspect that it is a mortar and will work fine but at a high
cost if you have to buy it.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net
------------------------------
Message: 12
Date: Sun, 18 Apr 2004 10:51:05 -0700
From: "Bob May" <bobmay@nethere.com>
Subject: Re: [ATM] Achromatic Doublet Design Help
To: "atml" <atm@atmlist.net>
Message-ID: <006b01c4256d$b93a86a0$6d823f42@p120>
I don't think that you're doing a F15 as the focal plane is 1200mm after the
50mm lens diameter. That makes it more of a F24 design!
I do believe that it is a diameter there rather than a radius.
The lens curves tend to be a bit shallow with a first curve of 1000mm and
the flint seems a bit odd with a near equal curvature on the two surfaces.
That lens is usually bent so that the back surface is near flat.
GSUM shows a quick starting point where the curves are
R1 909.6mm T1 10.67mm
R2 -574.6mm T2 .002mm
R3 -531mm T3 8.66mm
R4 -2245.8mm
and this looks a lot more typical of what the elements should look like.
Bob May
http://nav.to/bobmay
bobmay@nethere.com
NEW! http://bobmay.astronomy.net
------------------------------
Message: 13
Date: Sun, 18 Apr 2004 14:24:53 -0400
From: "Robert Turner" <rbturner@charter.net>
Subject: RE: [ATM] Achromatic Doublet Design Help
To: "atml" <atm@atmlist.net>
Message-ID: <JPENKNJKPIDNEHIGJNBOGECHCIAA.rbturner@charter.net>
Content-Type: text/plain; charset="iso-8859-1"
> I don't think that you're doing a F15 as the focal plane is
> 1200mm after the
> 50mm lens diameter. That makes it more of a F24 design!
> I do believe that it is a diameter there rather than a radius.
> The lens curves tend to be a bit shallow with a first curve of 1000mm and
> the flint seems a bit odd with a near equal curvature on the two surfaces.
> That lens is usually bent so that the back surface is near flat.
> GSUM shows a quick starting point where the curves are
> R1 909.6mm T1 10.67mm
> R2 -574.6mm T2 .002mm
> R3 -531mm T3 8.66mm
> R4 -2245.8mm
> and this looks a lot more typical of what the elements should look like.
> Bob May
Well after some help from list members I was able to read up some more and I
did the Gee Wyld program and came up with a better starting point. Then I
let OSLO optimize it. The result is a new design...
http://webpages.charter.net/astrorob/Robert_Achromat_2.len
The part I dont quite have a grasp for yet is how you determine the
thickness that will result starting from flat glass. I am going to start
from two pieces of glass square 11x11x3 cm. From which I will have to shape
into hopefully 4 inch lenses. Would I just rough in the curves to my
estimate and get an idea of my finish thickness first? Then go back with
another optimization in OSLO to determine what my final radius should be
based on what I end up with my rough cut?
I just have alot more to figure out then just going to someone and ordering
rough curves to my desired thickness. I have to start from square blocks of
glass.
Robert
------------------------------
Message: 14
Date: Sun, 18 Apr 2004 18:49:13 +0100
From: "Richard F.L.R. Snashall" <rflrs@rcn.com>
Subject: Re: [ATM] Achromatic Doublet Design Help
To: atml <atm@atmlist.net>
Message-ID: <4082BF99.2060804@rcn.com>
Content-Type: text/plain; charset=us-ascii; format=flowed
Robert Turner wrote:
>
>
> Well after some help from list members I was able to read up some more and I
> did the Gee Wyld program and came up with a better starting point. Then I
> let OSLO optimize it. The result is a new design...
>
> http://webpages.charter.net/astrorob/Robert_Achromat_2.len
>
You may want to go into "setup" in the lens data editor and set the
field angle to, say, 0.5 (degrees). That would give you an idea how
the lens performs off-axis as well.
> Robert
--
Rick S.
http://users.rcn.com/rflrs
------------------------------
Message: 15
Date: Sun, 18 Apr 2004 22:16:26 +0300
From: "Martti Koskimo" <martti.koskimo@kolumbus.fi>
Subject: Re: [ATM] Machine Polishing
To: <atm@atmlist.net>
Message-ID: <003701c42579$a5321590$6800a8c0@stellar19>
Content-Type: text/plain; charset="iso-8859-1"
There are so many variables influencing the figure.
What is the table rotating speed ? Is the tool rotating 'regularly' with a
bit lower speed than the table ? So it ought to do indicating that there
is good contact tool/mirror.
If you have too soft pitch every tools movement just a little over the edge
will unavoidable turn the edge down and it emphasize the hole in the middle
of the mirror
How are your machine moving the tool ? If the tool has very low profile
then it is ok but if the but if the driving pin is very high up the momentum
is unfavourable.
It should not be so very difficult to find adjustments for spherical
polishing so I guess that your pitch is too soft.
Martti Koskimo
----- Original Message -----
From: "aneves" <aneves@snowsoft.biz>
To: <atm@atmlist.net>
Sent: Sunday, April 18, 2004 8:07 PM
Subject: [ATM] Machine Polishing
> Hi,
>
> What is the correct way to spherical polish a mirror on a machine with a
sub diameter tool. Every method I try I end up with a hole in the middle!
Could some of your more experienced on machines give me some hints! How do
the pros do it? I am sure they must be able to pull
> a perfect sphere on a machine.
>
> I am doing an 8" with a 6" tool, 2" stroke offset by 2" to the right.
>
> Regards,
>
> Alfredo
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>
------------------------------
Message: 16
Date: Sun, 18 Apr 2004 16:39:31 -0400
From: "Tony Dunlap" <tdunlap@adelphia.net>
Subject: [ATM] re: ATM A new kind of folded Newtonian
To: <atm@atmlist.net>
Message-ID: <000001c42585$41bd7570$6400a8c0@dunlap01>
Content-Type: text/plain; charset="us-ascii"
Hello everyone.
Dredging up an old topic, I have a quick question.
Would the diagonal in the sketch below necessarily have to be as "flat"
as the diagonal in a typical Newtonian? It would seem to me not, since
the light hitting it is "bigger", but I'd like some opinions from those
more experienced. There seems to ba an additional advantage of no
spider. I haven't quite figured out how to cut an eliptical hole in it
though.
Thanks
Tony
>Jamie wrote:
> > Pertti,
> > Perhaps I'm missing something, but don't you have a standard folded
> > > Newtonian with an extra (unneeded) reflection?
> I've also encountered the standard folded Newtonian. See e.g.
> http://www.oldham-optical.co.uk/Astro.htm
> The differences between that design and mine are as follows:
> - significantly smaller central obstruction in mine,
> - lower eyepiece height attainable for a specified focal length and
> central obstruction in mine.
How about something like this: Basically a standard newtonian except the
diagonal is before the primary in the light path.
eye
Light----------\ |
|\
| | -eliptical hole
| \
Light---- -----|-|-\ <-45 degree flat
| |
| | |
| |
| | |
|___|
^
|
primary
------------------------------
Message: 17
Date: Sun, 18 Apr 2004 17:36:39 -0400
From: "Jerry" <wa4guu@verizon.net>
Subject: RE: [ATM] Achromatic Doublet Design Help
To: "'Robert Turner'" <rbturner@charter.net>, "'atml'"
<atm@atmlist.net>
Message-ID: <001901c4258d$3c260e80$0100a8c0@D85SJB21>
Content-Type: text/plain; charset="us-ascii"
The thickness is largely a mechanical "option". A bit of change in the
thicknesses will not have much effect on the design curves.
On the positive lens you want the center thickness to be sufficient
to give a good edge thickness to make mounting easy. On the negative lens
you want enough center thickness so that the glass doesn't flex too much in
grinding and polishing. The edge of the negative will naturally be thick
enough if the center is.
I made a 4" f16 which is negligibly different from f15. Starting
with 1" thick blanks, I made it about as thin as I would go if I were to do
it again. The positive is about .5 inch and the negative about .3 inch thick
at the centers. I didn't have any problem making these thicknesses, but if I
had made the crown much thinner the edge would be getting close to "too
thin". So maybe if you took those numbers and added a bit of a buffer so
that if you had to grind a bit more because of a scratch or something and
ended up thinner than intended, you would not "run out of glass". Around .6
or .7" for the positive and .4 or .5" for the negative wouldn't be bad
numbers to start with.
You can re-optimize when you get you finished (or should I say
roughly finished?) curves and thicknesses. But you will probably find that
you can't measure them to the precision necessary for re-optimization to be
effective. You will do well to measure your radii to a tenth inch accuracy.
The collective inaccuracies will probably cause the output of the
optimization program to be meaningless. In other words you should end up as
close as you can get to your numbers. If you re-optimize you end up with new
numbers that you will probably not be able to get any closer to than the
last set of numbers. So what is the point. Get as close as you can to your
design numbers. Any improvement will now have to be guided by optical
testing.
You could play with the thicknesses of the elements in OSLO. Vary them a bit
without re-optimizing and see how it affects the OSLO output. You can get a
feel for how much room for error you have. The same could be done with the
radii.
Jerry
------------------------------
Message: 18
Date: Mon, 19 Apr 2004 11:14:09 +1000
From: "Thomas" <tomjan@ozemail.com.au>
Subject: FW: [ATM] re: ATM A new kind of folded Newtonian
To: "ATM" <atm@atmlist.net>
Message-ID: <000901c425ab$b1f84650$fb01a8c0@mylaptop>
Content-Type: text/plain; charset="us-ascii"
I really should start using the reply all button;)
Clear skies,
Thomas Janstrom
http://www.tjanstrom.com
http://www.norsewines.com.au
-----Original Message-----
From: Thomas [mailto:tomjan@ozemail.com.au]
Sent: Monday, 19 April 2004 11:13 AM
To: 'Tony Dunlap'
Subject: RE: [ATM] re: ATM A new kind of folded Newtonian
For this kind of set up I think that the flatter the flat the better,
you would want it at least 1/10wave. You could use a good spherical
mirror but you would then have to change the figure of the primary to
match. This is actually the best way to go a it gives you another degree
of freedom in optimising the scope.
However if simplicity is your aim then the flatter the better.
Clear skies,
Thomas Janstrom
http://www.tjanstrom.com
http://www.norsewines.com.au
-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
Of Tony Dunlap
Sent: Monday, 19 April 2004 6:40 AM
To: atm@atmlist.net
Subject: [ATM] re: ATM A new kind of folded Newtonian
Hello everyone.
Dredging up an old topic, I have a quick question.
Would the diagonal in the sketch below necessarily have to be as "flat"
as the diagonal in a typical Newtonian? It would seem to me not, since
the light hitting it is "bigger", but I'd like some opinions from those
more experienced. There seems to ba an additional advantage of no
spider. I haven't quite figured out how to cut an eliptical hole in it
though.
Thanks
Tony
>Jamie wrote:
> > Pertti,
> > Perhaps I'm missing something, but don't you have a standard folded
> > > Newtonian with an extra (unneeded) reflection?
> I've also encountered the standard folded Newtonian. See e.g.
> http://www.oldham-optical.co.uk/Astro.htm
> The differences between that design and mine are as follows:
> - significantly smaller central obstruction in mine,
> - lower eyepiece height attainable for a specified focal length and
> central obstruction in mine.
How about something like this: Basically a standard newtonian except the
diagonal is before the primary in the light path.
eye
Light----------\ |
|\
| | -eliptical hole
| \
Light---- -----|-|-\ <-45 degree flat
| |
| | |
| |
| | |
|___|
^
|
primary
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------------------------------
Message: 19
Date: Sun, 18 Apr 2004 21:38:52 -0400
From: artbianconi@blast.net
Subject: [ATM] Couder Masks and Zone Testing
To: atm@atmlist.net
Cc: byron@astronomygeek.us
Message-ID: <4082F56C.1984.A5673A@localhost>
> I don't know how you can make a Couder mask with software. I always used
> black paper or cardboard and an X-acto knife.
>
> Note that it is best when testing to isolate the mask from the mirror.
> That way you can adjust the mask position without disturbing the distance
> from the mirror to the knife edge.
>
> . . . Richard
Bob May shows some photo's on his website of him using an inexpensive PC linked "eye-
ball" camera to capture the reflected image of the mirror at COC. He then projects the image
to a conventional CRT (TV set). Whether you are doing a Faucault test or a Ronchi, you
have the option of capturing the image, saving it to appropriate media and then using the
string of photos for checking your progress and adjusting your strokes.
Gordon Waite and Mike Lindner have taken the idea a little further. At the risk of
misrepresenting their creativity, I believe it goes something like this. . . . . .
they cut the mask not to the mirror but to fit the TV screen! Then they simply zoom in and
out until the mirror image on the screen is sized to fit the mask! Then they do their tests.
Presto! One mask fits all !
Neat Huh?
Art
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